It is well known in the art that silicon is a brittle substance. A penetration device constructed from single crystal silicon must possess a certain degree of mechanical robustness in order to ensure successful use of the needle without accidental fracture of the needle in patients. The device may have an interior channel (a hollow penetration device through which fluids can pass for sampling or injection) or it may be solid (for use as a lancet or probe). Integrated circuit and MEMS (microelectromechanical systems) technologies are used to fabricate these silicon penetration devices. Common MEMS fabrication methods, such as bulk etching with potassium hydroxide solution, leave the surface of the penetration device in a roughened state, with resultant increase in surface flaws. The actual failure of the silicon penetration device is the result of microcrack propagation initiated at a flaw on the surface of the material. It is important therefore, to increase the fracture toughness of the penetration device to permit reliable skin penetration without breakage.
In Kim et al, U.S. Statutory Invention Registration H001166, a tightly adherent thermally grown silicon containing oxide layer was utilized in order to limit the strength diminishing effects of microflaws located on the surface of the substrate. The materials applied are a composition of matter comprising silicon-based ceramics. In Kim, the material was exposed to an environment of essentially hydrogen and water vapor at the correct temperature and pressure for a predetermined amount of time thereby forming the oxide layer.
In Leger et al, U.S. Pat. No. 3,628,983, thin film coatings were applied to vitreous and vitrocrystalline (derived from or consisting of glass) bodies. Chemical modifications were made to the coatings of Leger while the coatings were in a heated condition with the film and substrate being subsequently cooled to create compressive stresses in the films. The chemical modifications varied between a step involving the oxidation of a metal or a metal compound, and the replacement of alkali metal ions in the coating by ions which derived from the medium and which conferred on the coating a lower coefficient of thermal expansion. Creating a compressive stress on the surface of the substrate provides a means of holding together surface defects which otherwise limit the strength of the material.
In Ishi et al, U.S. Pat. No. 4,985,368, the substrate of a semiconductor was strengthened by depositing oxide over a corner of the device. The substrate consisted of a main surface, a predetermined impurity concentration of a first conductivity type, and a trench with a sufficient radius of curvature over at least a bottom corner portion thereof. A two-layer film consisting of oxide and nitride was formed on the main surface of the substrate, the side portions of the trench and a portion of the bottom of the trench. A second, selective, oxide layer was formed on the bottom and at the corner portion of the trench. The selective oxide layer spanned the corner portion of the trench with a radius of curvature more than {fraction (1/10)} and less than ½ of the width of the trench.
The above prior art teaches the application of oxide to produce a compressive film intended to reduce microcrack propagation. In no case was the film subsequently removed, providing the substantial increase in fracture toughness shown in our present invention.
In Leger, an external thin film coating was applied to a material in an attempt to minimize the effects of surface flaws. However, Leger did not use nickel as one of the thin films. Furthermore, the materials that are being strengthened are vitreous and vitrocrystalline bodies. Also, the methods of Leger involve creating compressive stresses in the films through both chemical treating at an elevated temperature and subsequent cooling.
In Wilson et al “Fracture Testing of Bulk Silicon Microcantilever Beams Subjected to a Side Load” silicon microcantilevers were fracture tested experimentally. However, the silicon microcantilevers of the Wilson experiment did not undergo surface modifications. Furthermore, in Wilson, the strengthening of devices through surface modifications was not addressed.
While the prior art devices exemplify existing methods, there still exists a need for improved methods for strengthening silicon penetration devices by increasing fracture toughness.